theorem Th17:
  for D be non empty set holds the_unity_wrt addpfunc(D) = [#](D)
  --> (0 qua Real)
proof
  let D be non empty set;
  [#](D) --> In(0,REAL) is_a_unity_wrt addpfunc(D) by Th16;
  hence thesis by BINOP_1:def 8;
end;
